Preheat the oven to 400 degrees F (200 degrees C/Gas 6).
Mix the butter with the sun-dried tomatoes, chile, garlic, and thyme leaves. Season and spoon over each mushroom. Bake for 10 to 15 minutes, or until soft.
Cut the sourdough into 8 slices and spread with the Dijon mustard. Lay the watercress over the mustard and top with the baked mushrooms. Cover with the other half of bread, press down firmly, and cut.